Bearing steel

ABSTRACT

A bearing steel contains C: 0.56% by mass or more and 0.70% by mass or less, Si: 0.15% by mass or more and less than 0.50% by mass, Mn: 0.60% by mass or more and 1.50% by mass or less, Cr: 0.50% by mass or more and 1.10% by mass or less, P: 0.025% by mass or less, S: 0.025% by mass or less, Al: 0.005% by mass or more and 0.500% by mass or less, O: 0.0015% by mass or less, N: 0.0030% by mass or more and 0.015% by mass or less, and a remainder of Fe and incidental impurities. The bearing steel has a composition such that the eutectic carbide formation index Ec satisfies 0&lt;Ec≦0.25.

RELATED APPLICATIONS

This is a §371 of International Application No. PCT/JP2010/071778, withan international filing date of Nov. 30, 2010 (WO 2011/065592, publishedJun. 3, 2011), which is based on Japanese Patent Application No.2010-242668, filed Oct. 28, 2010, and Japanese Patent Application No.2009-272929, filed Nov. 30, 2009, the subject matter of which isincorporated by reference.

TECHNICAL FIELD

This disclosure relates to a bearing steel having excellent rollingcontact fatigue life characteristics and suitable as a bearing materialfor use in automobiles, wind power, transport machines, electricalmachines, precision machines, and other general industrial machinery.

BACKGROUND

High-carbon chromium steel (JIS G4805 standard SUJ2) has been widelyused as a bearing steel of this type. In general, one of the importantproperties of bearing steels is excellent rolling contact fatigue lifecharacteristics. It is generally believed that the rolling contactfatigue life of steel is shortened by the presence of non-metallicinclusions or eutectic carbide in the steel.

It has been believed in recent studies that non-metallic inclusions insteel are mainly responsible for the decrease in rolling contact fatiguelife. Thus, bearing life has been improved by reducing the oxygencontent of steel to control the number and size of non-metallicinclusions in the steel.

For example, Japanese Unexamined Patent Application Publication Nos.1-306542 and 3-126839 propose techniques for controlling thecomposition, shape, or distribution of an oxide-based non-metallicinclusion in steel. However, the manufacture of a bearing steelcontaining a decreased number of non-metallic inclusions requiresexpensive converter steelmaking machines or extensive modifications ofconventional facilities, which entails an immense economic burden.

Japanese Unexamined Patent Application Publication No. 7-127643discloses a technique for improving the rolling contact fatigue lifecharacteristics by controlling the centerline segregation rate of carbonand the oxygen and sulfur contents of steel. However, as describedabove, a further reduction in oxygen content to manufacture a bearingsteel containing a decreased number of non-metallic inclusions requiresexpensive converter steelmaking machines or extensive modifications ofconventional facilities, which entails an immense economic burden.

Thus, not only the decrease in the number of non-metallic inclusions insteel, but also the decrease in the eutectic carbide content of steelhave received attention. For example, although a high-carbon chromiumsteel, which contains 0.95% by mass or more C, is very hard and has highwear resistance, the central portion of a casting steel has a highdegree of segregation (hereinafter referred to as “centerlinesegregation”). Furthermore, enormous eutectic carbide is formed in thecasting steel, which shortens the rolling contact fatigue life. Thus,the central portion of the casting steel is removed as waste orsubjected to diffusion treatment (hereinafter referred to as “soaking”)for a long period of time to sufficiently eliminate the centerlinesegregation and the eutectic carbide.

Regarding such a segregation problem, Japanese Patent No. 3007834discloses a method in which steel is controlled to have a specificcomposition such as C: 0.6% to 1.2% by mass, and the totalcross-sectional area of carbide having a thickness of 2 μm or more andwith respect to the center line of a longitudinal cross section throughthe shaft center of a wire or rod-shaped rolled steels, existing in acentral region within ⅛×D (D: the width of the longitudinal section)from the axis, including the axis of the longitudinal section, is 0.3%or less of the area of the longitudinal section. Japanese Patent No.3007834 also quantitatively shows the influence of the amount ofenormous carbide on the rolling contact fatigue life characteristics,indicating the presence of enormous eutectic carbide in steel thatshortens the rolling contact fatigue life.

Japanese Unexamined Patent Application Publication No. 5-271866discloses a bearing steel that has a specific composition such as C:0.50% to 1.50% by mass and Sb: 0.0010% to 0.0150% by mass, a decreasedamount of decarburized layer, and high thermal process productivity. Itis an object of Japanese Unexamined Patent Application Publication No.5-271866 to improve thermal process productivity by the addition of Sbto decrease formation of a decarburized layer in steel and therebyeliminate the cutting or grinding process after the thermal process.However, Sb may be highly toxic to humans and should therefore betreated carefully. Furthermore, addition of Sb results in thecondensation of Sb in the central segregation zone, worsening centerlinesegregation. A portion containing condensed Sb can be locally hardenedto have a different hardness from the base material. The differenthardness may induce rolling contact fatigue fracture, shortening therolling contact fatigue life.

To eliminate centerline segregation in the casting of a high-carbonchromium bearing steel and enormous eutectic carbide formed in thecentral segregation zone, for example, Japanese Unexamined PatentApplication Publication No. 3-75312 discloses a method for rolling acasting material into a billet and soaking the billet.

However, because of nonuniform steel temperature in soaking, a soakingtemperature exceeding the solidus line in a portion could initiatemelting in the portion again and induce a eutectic reaction, formingfurther enormous eutectic carbide.

Thus, in some applications of bearings, low-carbon alloy steel may beused in place of high-carbon chromium steel. For example, case-hardenedsteel is most commonly used after high-carbon chromium steel. However,case-hardened steel contains 0.23% by mass or less C and moderateamounts of Mn, Cr, Mo, and Ni or the like to achieve necessary quenchhardenability and mechanical strength. The surface of case-hardenedsteel is hardened by carburization or carbonitriding to improve fatiguestrength.

For example, Japanese Patent No. 4066903 discloses a case-hardened steelthat can be carburized in a short period of time, wherein thecase-hardened steel has a specific chemical composition, such as C:0.10% to 0.35%, and the activation energy of carbon diffusion in thesteel defined by Q=34140−605[% Si]+183[% Mn]+136[% Cr]+122[% Mo] is34000 kcal or less.

Likewise, Japanese Patent No. 4050829 discloses a technique regarding acarburized material that has excellent rolling contact fatiguecharacteristics, wherein the carburized material has a specific chemicalcomposition such as C: 0.1% to 0.45%, the austenite grain size number ofa carburized layer is 7 or more, the carbon content of the surfaceranges from 0.9% to 1.5%, and the retained austenite content of thesurface ranges from 25% to 40%.

Although carburization or carbonitriding can improve the rolling contactfatigue life characteristics, it may increase the manufacturing costs ordecrease the yield because of a large strain or dimension change, thusincreasing product cost.

Some applications of bearing steels require a large section. Thisrequires extensive modifications of carburization or carbonitridingfacilities, which entail an immense economic burden.

With a year-by-year increase in the scale of wind power, transportmachines, and general industrial machinery, there has been an urgentneed to further increase the section of bearing steels. Methods formanufacturing steel ingots are broadly divided into ingot casting andcontinuous casting. Bearing steels having a small section to a largesection, which are hitherto manufactured by continuous casting steel,can be provided by manufacturing ingot casting steel to answer theupsizing tendency. However, steels manufactured by the ingot casting(hereinafter referred to as “ingot steels”) have a particular problemthat enormous eutectic carbide is formed in a segregation zone such as aV-segregation zone or an inverse V-segregation zone. This is becauseingot steels have a higher degree of segregation and consequently ahigher frequency of enormous eutectic carbide than continuously castingsteels. Thus, it is important to decrease the formation of eutecticcarbide.

Accordingly, it could be helpful to provide a method for decreasingformation of eutectic carbide in the segregation zone in bearing steelsparticularly made of ingot steels, as well as continuously castingsteels.

SUMMARY

We discovered that the amounts of C, Si, Mn, Cr, and Al added to aconventional bearing steel are limited to a specific range and aeutectic carbide formation index is newly introduced and limited to aspecific range. Thus, we found that these limitations can avoidformation of enormous eutectic carbide in the V-segregation zone or theinverse V-segregation zone, which are particularly problematic in ingotsteels, and a bearing steel having excellent rolling contact lifecharacteristics can be provided.

More specifically, we manufactured a bearing steel made of an ingotsteel in which the amounts of C, Si, Mn, Cr, and Al are altered and theeutectic carbide formation index Ec having the formula (1) describedbelow is altered. As a result of extensive studies on the structure andthe rolling contact fatigue life characteristics of the bearing steel,we found that a steel, even made of an ingot steel, having a compositionand Ec in specific ranges can be free of eutectic carbide in the steeland have improved rolling contact fatigue life characteristics.

We thus provide:

1. A bearing steel having a composition containing

C: 0.56% by mass or more and 0.70% by mass or less,

Si: 0.15% by mass or more and less than 0.50% by mass,

Mn: 0.60% by mass or more and 1.50% by mass or less,

Cr: 0.50% by mass or more and 1.10% by mass or less,

P: 0.025% by mass or less,

S: 0.025% by mass or less,

Al: 0.005% by mass or more and 0.500% by mass or less,

O: 0.0015% by mass or less,

N: 0.0030% by mass or more and 0.015% by mass or less, and

a remainder of Fe and incidental impurities,

wherein eutectic carbide formation index Ec defined by the formula (1)satisfies

0<Ec≦0.25,

wherein

Ec=(−0.07×[% Si]−0.03×[% Mn]+0.04×[% Cr]−0.36×[% Al]+0.79)−[% C]  (1)

wherein [ ] indicates the amount of component described in parentheses(% by mass).

2. The bearing steel according to 1 described above, wherein thecomposition further contains one or more selected from

Cu: 0.005% by mass or more and 0.5% by mass or less,

Ni: 0.005% by mass or more and 1.00% by mass or less, and

Mo: 0.01% by mass or more and 0.5% by mass or less.

3. The bearing steel according to 1 or 2 described above, wherein thecomposition further contains one or more selected from

W: 0.001% by mass or more and 0.5% by mass or less,

Nb: 0.001% by mass or more and 0.1% by mass or less,

Ti: 0.001% by mass or more and 0.1% by mass or less,

Zr: 0.001% by mass or more and 0.1% by mass or less, and

V: 0.002% by mass or more and 0.5% by mass or less.

4. The bearing steel according to any one of 1 to 3 described above,wherein the composition further contains

B: 0.0002% by mass or more and 0.005% by mass or less.

Summarizing these aspects 1 to 4, a bearing steel has a compositioncontaining

C: 0.56% by mass or more and 0.70% by mass or less, Si: 0.15% by mass ormore and less than 0.50% by mass, Mn: 0.60% by mass or more and 1.50% bymass or less, Cr: 0.50% by mass or more and 1.10% by mass or less, P:0.025% by mass or less, S: 0.025% by mass or less, Al: 0.005% by mass ormore and 0.500% by mass or less, O: 0.0015% by mass or less, and N:0.0030% by mass or more and 0.015% by mass or less,

and optionally at least one of (A) to (C):

(A) one or more selected from Cu: 0.005% by mass or more and 0.5% bymass or less, Ni: 0.005% by mass or more and 1.00% by mass or less, andMo: 0.01% by mass or more and 0.5% by mass or less,

(B) one or more selected from W: 0.001% by mass or more and 0.5% by massor less, Nb: 0.001% by mass or more and 0.1% by mass or less, Ti: 0.001%by mass or more and 0.1% by mass or less, Zr: 0.001% by mass or more and0.1% by mass or less, and V: 0.002% by mass or more and 0.5% by mass orless, and

(C) B: 0.0002% by mass or more and 0.005% by mass or less,

and a remainder of Fe and incidental impurities,

wherein eutectic carbide formation index Ec defined by the formula (I)satisfies 0<Ec≦0.25.

A bearing steel having much better rolling contact fatigue lifecharacteristics than conventional bearing steels can be stablymanufactured. In particular, ingot steels can be used to manufacturebearing steels having a small section to those having a large section.Our bearing steels also contribute to upsizing of wind power generators,transport machines, and general industrial machinery, providingindustrially advantageous effects.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a graph showing the evaluation result of the rolling contactfatigue life (vertical axis: B₁₀ life ratio) as a function of Ec(horizontal axis: % by mass).

FIG. 2 is a view illustrating the sampling position and the test surfacesize in sampling for microstructure observation from a steel billetafter square forging.

FIG. 3 is a view illustrating the sampling position and the testspecimen size in sampling for the evaluation of rolling contact lifefrom a steel billet after square forging.

FIG. 4 is a view illustrating the sampling position and the test surfacesize in sampling for microstructure observation from a steel billetafter circular forging.

FIG. 5 is a view illustrating the sampling position and the test surfacesize in sampling for the evaluation of rolling contact life from a steelbillet after circular forging.

FIG. 6 is a view illustrating the sampling position and the testspecimen size in sampling for the evaluation of machinability from asteel billet after square forging.

FIG. 7 is a view illustrating the sampling position and the testspecimen size in sampling for the evaluation of machinability from asteel billet after circular forging.

DETAILED DESCRIPTION

Our bearing steels will be described in detail below.

First, reasons for limiting the percentage of each component of thecomposition of the bearing steel will be described below.

C: 0.56% by mass or more and 0.70% by mass or less

C can increase the strength of steel and effectively improve the rollingcontact fatigue life characteristics of steel. The C content is 0.56% bymass or more. On the other hand, the C content of more than 0.70% bymass results in the formation of enormous eutectic carbide during thecasting of the material, shortening the rolling contact fatigue life.Thus, the C content is 0.56% by mass or more and 0.70% by mass or less.

Si: 0.15% by mass or more and less than 0.50% by mass

Si can act as a deoxidizing agent, increase the strength of steel owingto solid-solution hardening, and improve the rolling contact fatiguelife characteristics of steel. Si is added to produce these effects.0.15% by mass or more Si is added to produce these effects. However,addition of 0.50% by mass or more Si results in deterioration in themachinability and the forgeability of steel. Si can be bound to oxygenin steel and remain as an oxide in the steel, causing deterioration inthe rolling contact fatigue life characteristics. Furthermore, Sicondensed in a segregation zone facilitates formation of eutecticcarbide. Thus, the upper limit of Si is less than 0.50% by mass.

Mn: 0.60% by mass or more and 1.50% by mass or less

Mn can be added to improve quench hardenability, increase the toughnessof steel, and improve the rolling contact fatigue life characteristicsof steel. 0.60% by mass or more Mn is added. However, the addition ofmore than 1.50% by mass Mn results in deterioration in machinability.Furthermore, Mn condensed in a segregation zone facilitates formation ofeutectic carbide. Thus, the upper limit of Mn is 1.50% by mass.

Cr: 0.50% by mass or more and 1.10% by mass or less

In the same manner as in Mn, Cr can be added to increase the toughnessof steel and improve the rolling contact fatigue life characteristics ofsteel. 0.50% by mass or more Cr is added. However, addition of more than1.10% by mass Cr results in deterioration in machinability. Thus, theupper limit of Cr is 1.10% by mass.

P: 0.025% by mass or less

P is a detrimental element that can decrease the base material toughnessor the rolling contact fatigue life of steel and is preferably decreasedas much as possible. In particular, the P content of more than 0.025% bymass results in a significant decrease in base material toughness androlling contact fatigue life. Thus, the P content is 0.025% by mass orless, preferably 0.020% by mass or less. It is industrially difficult toachieve the P content of 0%. Thus, the P content is often 0.003% by massor more.

S: 0.025% by mass or less

S is contained in steel as a non-metallic inclusion MnS. Since bearingsteels contain a decreased amount of oxide, which can often inducerolling contact fatigue, a large amount of MnS in steel can shorten therolling contact fatigue life. Thus, S is preferably decreased as much aspossible. The S content is 0.025% by mass or less, preferably 0.020% bymass or less. It is industrially difficult to achieve the S content of0%. Thus, the S content is often 0.0001% by mass or more.

Al: 0.005% by mass or more and 0.500% by mass or less

Al can act as a deoxidizing agent, form a nitride and decrease the sizeof austenite grains, and improve the toughness and the rolling contactfatigue life characteristics. Al is added to produce these effects.0.005% by mass or more Al is added to produce these effects. However,addition of more than 0.500% by mass Al results in formation of a coarseoxide type inclusion in steel, causing deterioration in the rollingcontact fatigue life characteristics of steel. Furthermore, Al condensedin a segregation zone facilitates formation of eutectic carbide. Thus,the upper limit of the Al content is 0.500% by mass, preferably 0.450%by mass or less.

O: 0.0015% by mass or less

O can be bound to Si or Al to form a hard oxide-based non-metallicinclusion, shortening the rolling contact fatigue life. Thus, O ispreferably decreased as much as possible and is 0.0015% by mass or less.It is industrially difficult to achieve an O content of 0%. Thus, the Ocontent is often 0.0003% by mass or more.

N: 0.0030% by mass or more and 0.015% by mass or less

N can be bound to Al to form a nitride-based non-metallic inclusion,decrease the size of austenite grains, and improve the toughness and therolling contact fatigue life characteristics. Thus, 0.003% by mass ormore N is added. However, the addition of more than 0.015% by mass Nresults in formation of a large number of nitride-based inclusions insteel, causing deterioration in the rolling contact fatigue lifecharacteristics. This also results in the presence of a large amount ofN that does not form a nitride in steel (free N), thus decreasing thetoughness of steel. Thus, the upper limit of the N content is 0.015% bymass, preferably 0.010% by mass or less.

Eutectic carbide formation index Ec: 0<Ec≦0.25

We smelted steels having various compositions in a vacuum meltingfurnace. The resulting steel ingot was examined for the presence ofeutectic carbide. Regression calculation on the result was performedwith various selected sets of parameters (main influential elements). Asa result, it was found that the eutectic carbide index Ec defined by theformula (1) must satisfy 0<Ec≦0.25 as a steel composition with whichformation of eutectic carbide can be decreased.

Ec=(−0.07×[% Si]−0.03×[% Mn]+0.04×[% Cr]−0.36×[% Al]+0.79)−[% C]  (1)

wherein [ ] indicates the amount of component described in parentheses(% by mass).

We manufactured bearing steels having the compositions and Ec's listedin Table 1 and examined their rolling contact fatigue lifecharacteristics. The rolling contact fatigue life characteristics wereperformed by the test method described below in the examples.

Bearing steels were manufactured under fixed conditions to examine thepresence of eutectic carbide and the influence of the composition and Econ the rolling contact fatigue life characteristics. More specifically,after smelting in a converter, an ingot steel (ingot) having a 1350mm×1250 mm section (top side) and a 1280×830 mm section (bottom side)was formed by ingot casting. The ingot steel was forged to have a 550 mmsquare section. Test specimens for observing formed eutectic carbideillustrated in FIG. 2 and rolling contact fatigue test specimensillustrated in FIG. 3 were sampled from the forged steel billet. Thepresence of eutectic carbide, the rolling contact fatigue lifecharacteristics, and machinability (determined from the tool life ratio)were examined by the test method described below.

Each of the test specimens was sampled from a portion of the forgedsteel billet corresponding to the bottom of the ingot steel.

TABLE 1 (% by mass) Steel No. C Si Mn P S Cr Al O N Ec Note A-1 1.050.25 0.45 0.016 0.008 1.45 0.025 0.0010 0.0031 −0.24 Reference steel A-20.70 0.46 1.23 0.018 0.005 0.51 0.021 0.0011 0.0030 0.03 Inventive steelA-3 0.70 0.49 1.50 0.011 0.005 0.51 0.121 0.0010 0.0038 −0.01Comparative steel A-4 0.62 0.32 0.97 0.010 0.006 0.88 0.024 0.00080.0052 0.15 Inventive steel A-5 0.57 0.16 0.74 0.015 0.003 0.71 0.0210.0009 0.0049 0.21 Inventive steel A-6 0.48 0.19 0.58 0.011 0.005 1.090.033 0.0009 0.0045 0.31 Comparative steel A-7 0.56 0.22 0.62 0.0080.002 1.00 0.028 0.0008 0.0036 0.23 Inventive steel A-8 0.55 0.23 0.630.007 0.003 0.95 0.021 0.0009 0.0042 0.24 Comparative steel A-9 0.560.16 0.60 0.007 0.003 1.00 0.037 0.0007 0.0035 0.23 Inventive steel A-100.56 0.16 0.58 0.009 0.003 1.05 0.036 0.0008 0.0031 0.23 Comparativesteel A-11 0.70 0.50 1.50 0.012 0.003 0.51 0.340 0.0011 0.0042 −0.09Comparative steel A-12 0.67 0.39 0.95 0.008 0.002 0.56 0.035 0.00100.0035 0.07 Inventive steel

Table 2 shows the evaluation results of rolling contact fatigue life andmachinability (determined from the tool life ratio). FIG. 1 shows therelationship between the evaluation results of rolling contact fatiguelife (vertical axis: B₁₀ life ratio) and Ec (horizontal axis: % bymass). As shown in that figure, enormous eutectic carbide is formed insteel when Ec is 0 or less. Even an increase in Ec in this range cannotsignificantly improve the rolling contact fatigue life relative to thelevel of a reference material. When Ec is more than 0, eutectic carbideis not formed, and the rolling contact fatigue life is markedlyimproved. However, Ec of more than 0.25 resulted in a decrease in theamount of C added and consequently a decrease in the strength and therolling contact fatigue life of steel after quenching. Thus, when Ec isin the range of 0<Ec≦0.25, eutectic carbide is not formed in steel, andtherefore the rolling contact fatigue life characteristics are improved.Even when Ec is within our range, A-8 having a C content outside ourrange and A-10 having a Mn content outside our range had a decreasedstrength and consequently a shortened rolling contact fatigue life.

TABLE 2 Test results Presence of Rolling contact Steel eutectic fatiguelife Tool life Symbol in No. carbide Ec (B10 life ratio) ratio Note FIG.1 A-1 Yes −0.24 1.00 1.00 Reference steel  A-2 No 0.03 1.16 1.21Inventive steel ♦ A-3 Yes −0.01 1.06 1.20 Comparative steel ♦ A-4 No0.15 1.32 1.22 Inventive steel ♦ A-5 No 0.21 1.25 1.24 Inventive steel ♦A-6 No 0.31 1.07 1.24 Comparative steel ▪ A-7 No 0.23 1.17 1.20Inventive steel ♦ A-8 No 0.24 1.08 1.20 Comparative steel ▴ A-9 No 0.231.16 1.21 Inventive steel ♦ A-10 No 0.23 1.09 1.20 Comparative steel ▴A-11 Yes −0.09 1.03 1.18 Comparative steel ♦ A-12 No 0.07 1.23 1.22Inventive steel ♦

The reason for limiting Ec to produce a steel free of eutectic carbideis that, as described above, formation of eutectic carbide in steel cancause rolling contact fatigue originating from the eutectic carbide,which causes deterioration in the rolling contact fatigue lifecharacteristics.

Formation of eutectic carbide can be decreased even with an ingot steelmanufactured by ingot casting. Thus, our concepts are particularlyeffective when applied to ingot steels manufactured by ingot casting. Itis also effective to use ingot steels to manufacture bearing productshaving a small section to those having a large section.

In addition to these base components, the following components canappropriately be added.

(A) One or more selected from Cu: 0.005% to 0.5% by mass, Ni: 0.005% to1.00% by mass, and Mo: 0.01% to 0.5% by mass

Cu, Ni, and Mo can improve the quench hardenability, the strength aftertempering, and the rolling contact fatigue life characteristics of steeland can be selectively added in accordance with the strength required(more specifically, any one of Cu, Ni, Mo, Cu+Ni, Cu+Mo, Ni+Mo, andCu+Ni+Mo can be selectively added). The amounts to be added arepreferably 0.005% by mass or more for Cu and Ni and 0.01% by mass ormore for Mo to produce such effects. However, addition of more than 0.5%by mass Cu or Mo or more than 1.00% by mass Ni results in deteriorationin machinability of the steel. Thus, Cu, Ni, and Mo are preferably addedin amounts equal to or below these upper limits.

Likewise, to increase the strength or improve the rolling contactfatigue life characteristics of steel, in addition to the componentsdescribed above, the following components may be added to our bearingsteel.

(B) One or more of W: 0.001% to 0.5% by mass, Nb: 0.001% to 0.1% bymass, Ti: 0.001% to 0.1% by mass, Zr: 0.001% to 0.1% by mass, and V:0.002% to 0.5% by mass

W, Nb, Ti, Zr, and V can improve the quench hardenability, the strengthafter tempering, and the rolling contact fatigue life characteristics ofsteel and can be selectively added in accordance with the strengthrequired (more specifically, any one of W, Nb, Ti, Zr, V, W+Nb, W+Ti,W+Zr, W+V, Nb+Ti, Nb+Zr, Nb+V, Ti+Zr, Ti+V, Zr+V, W+Nb+Ti, W+Nb+Zr,W+Nb+V, W+Ti+Zr, W+Ti+V, W+Zr+V, Nb+Ti+Zr, Nb+Ti+V, Nb+Zr+V, Ti+Zr+V,W+Nb+Ti+Zr, W+Nb+Ti+V, W+Nb+Zr+V, W+Ti+Zr+V, Nb+Ti+Zr+V, andW+Nb+Ti+Zr+V can be selectively added). The amounts to be added arepreferably 0.001% by mass or more for W, Nb, Ti, and Zr and 0.002% bymass or more for V to produce such effects. However, addition of morethan 0.5% by mass W or V or more than 0.1% by mass of Nb, Ti, or Zrresults in deterioration in machinability of the steel. Thus, theseelements are preferably added in amounts equal to or below these upperlimits.

(C) B: 0.0002% to 0.005% by mass

B can improve quench hardenability and thereby increase the strength ofsteel after tempering and improve the rolling contact fatigue lifecharacteristics of steel. B can be added to steel on an as-needed basis0.0002% by mass or more of B is preferably added to produce theseeffects. However, addition of more than 0.005% by mass B results indeterioration in workability. Thus, 0.0002% to 0.005% by mass B ispreferably added.

When an element other than the base components is added, any combinationof elements of (A), (B), and (C) groups is available. More specifically,an element selected from any one element group may be added, elementsselected from each of two element groups may be added, or elementsselected from each of all the element groups may be added.

In our bearing steel, the components other than the components describedabove are Fe and incidental impurities. Examples of the incidentalimpurities include, but are not limited to, Sn, Sb, As, and Ca.

A bearing steel having the composition described above is smelted in avacuum melting furnace or a converter and further by a known refiningmethod such as a degassing process, and is then formed into a castbillet by ingot casting or continuous casting. Even when a cast billetis formed by ingot casting by which eutectic carbide is particularlyeasily precipitated, formation of eutectic carbide can be prevented.Thus, our concepts can be applied to ingot steels (with which large castbillets can be manufactured). Cast billets are subsequently subjected toa forming process, such as rolling or forging, to produce bearingcomponents.

EXAMPLES Example 1

A steel having a composition listed in Table 3 was smelted by converterrefining and a degassing process and then formed into a cast billethaving a size listed in Table 4 by ingot casting or continuous casting.The cast billet was heated to a temperature of 1000° C. to 1350° C. in afurnace and then forged to have a section size listed in Table 4. Thepresence of eutectic carbide and the rolling contact fatigue lifecharacteristics of the forged product were examined as described below.

Presence of Eutectic Carbide

The presence of eutectic carbide was examined by taking a sample formicrostructure observation from a (T₁/2, T₂/2) portion (central portion)and a (T₁/2, T₂/4) portion (T₁=T₂ denote a side length of asquare-forged steel billet: FIG. 2) or a D/4 portion and a D/2 portion(D denotes the diameter of a circular-forged steel billet: FIG. 4) of aforged steel billet such that the section in the drawing directionbecame a surface to be observed, etching the sample with 3% nital, andobserving the sample with a scanning electron microscope (SEM) at amagnification ratio of 500. The test area was 10 mm×10 mm. Each testspecimen was sampled from a portion of the forged steel billetcorresponding to the bottom of an ingot steel.

Rolling Contact Fatigue Life Characteristics

The rolling contact fatigue life characteristics are preferablydetermined in actual use after forging, cutting, quenching, andtempering. However, this requires a long term. Thus, the rolling contactfatigue life characteristics were determined with a thrust type rollingcontact fatigue machine as described below. A 60 mmφ×5.3 mm disk was cutfrom a (T₁/2, T₂/4) portion (T₁=T₂ denote a side length of asquare-forged steel billet: FIG. 3) or a D/4 portion (D denotes thediameter of a circular-forged steel billet: FIG. 5) of a forged steelbillet, was heated at 950° C. for 20 minutes, and was quenched with anoil at 25° C. The disk was then tempered at 170° C. for 1.5 hours andwas flat-polished to 60 mmφ×5 mm. The test surface was mirror-finished.The test specimen thus prepared was subjected to a rolling contactfatigue test under a maximum Hertzian contact stress of 5.8 GPa with athrust rolling contact fatigue machine in which a steel ball rolled onthe circumference of a circle having a diameter of approximately 38 mm,Each of the test specimens was sampled from a portion of the forgedsteel billet corresponding to the bottom of an ingot steel or acontinuously casting steel.

The rolling contact fatigue life characteristics were determined asdescribed below. The stress loading frequency when the test specimenunderwent detachment was measured for 10 to 15 test specimens. Therelationship between the cumulative probability and the stress loadingfrequency was organized using Weibull probability paper. After that, thecumulative probability 10% (hereinafter referred to as B₁₀ life) wasdetermined. The rolling contact fatigue life characteristics were judgedto be improved when the B₁₀ life was improved by 10% or more withrespect to a reference steel (A-1: SUJ2 equivalent steel).

Machinability

Machinability is preferably determined in actual processing afterforging, cutting, quenching, and tempering. However, this requires along term. Thus, machinability was determined in a turning test (latheturning test of the outer surface) as described below. A 60 mmφ×270 mmround bar was cut from a (T₁/2, T₂/4) portion (T₁=T₂ denote a sidelength of a square-forged steel billet: FIG. 6) or a D/4 portion (Ddenotes the diameter of a circular-forged steel billet: FIG. 7) of aforged steel billet, was heated at 950° C. for 20 minutes and quenchedwith an oil at 25° C. The bar was then tempered at 170° C. for 1.5hours. Machinability of the test specimen thus prepared was determinedwith a lathe turning tester. The turning test was performed with asuperhard (P10) cutting tool without a lubricant at a cutting speed of120 mm/min, a feed speed of 0.2 m/rev, and a depth of cut of 1.0 mm. Theamount of time elapsed to the time the flank wear of the tool was 0.2 mmwas considered to be the tool life. The degree of reduction in life(tool life ratio=tool life/tool life of SUJ2 equivalent steel) wasdetermined by dividing the tool life for each steel by the tool life forthe reference steel (A-1: SUJ2 equivalent steel). Machinability wasjudged to be improved when the tool life ratio was improved by 15% ormore with respect to the reference steel.

TABLE 3 Chemical components (% by mass) Steel No. C Si Mn P S Cr Al O NCu Ni A-1 1.05 0.25 0.45 0.016 0.008 1.45 0.025 0.0010 0.0031 — — B-10.62 0.33 0.98 0.010 0.006 0.87 0.025 0.0010 0.0033 0.01 0.01 B-2 0.600.26 0.85 0.011 0.005 0.80 0.025 0.0009 0.0031 0.02 0.02 B-3 0.69 0.451.44 0.015 0.006 0.53 0.490 0.0011 0.0041 — — B-4 0.56 0.42 1.00 0.0120.005 0.52 0.005 0.0010 0.0035 0.24 0.75 B-5 0.69 0.16 0.61 0.015 0.0090.94 0.031 0.0009 0.0042 — — B-6 0.59 0.44 1.43 0.012 0.006 0.93 0.0200.0008 0.0043 — — B-7 0.70 0.49 1.53 0.011 0.005 0.51 0.111 0.00090.0087 — — B-8 0.60 0.24 0.97 0.010 0.007 0.95 0.025 0.0014 0.0042 0.210.11 B-9 0.47 0.45 1.48 0.011 0.009 0.96 0.029 0.0011 0.0049 — — B-100.92 0.21 0.77 0.015 0.006 0.95 0.033 0.0010 0.0044 — — B-11 0.62 0.700.92 0.011 0.005 0.72 0.032 0.0011 0.0062 0.18 0.09 B-12 0.69 0.49 1.490.012 0.006 0.50 0.211 0.0009 0.0042 — — B-13 0.63 0.25 1.20 0.011 0.0060.89 0.049 0.0005 0.0081 — — B-14 0.70 0.33 0.97 0.009 0.007 0.51 0.0050.0007 0.0055 — — B-15 0.57 0.49 1.44 0.008 0.006 0.52 0.022 0.00090.0042 — — B-16 0.69 0.21 0.61 0.015 0.009 0.93 0.035 0.0007 0.0041 — —B-17 0.65 0.49 1.12 0.011 0.008 0.55 0.005 0.0009 0.0059 — — B-18 0.560.16 1.05 0.015 0.008 1.10 0.080 0.0009 0.0059 — — B-19 0.57 0.17 0.730.012 0.007 0.72 0.010 0.0007 0.0045 0.01 0.02 B-20 0.63 0.25 0.88 0.0130.007 0.81 0.530 0.0007 0.0045 0.01 0.02 B-21 0.63 0.34 0.89 0.009 0.0030.83 0.026 0.0008 0.0039 — — B-22 0.56 0.35 0.99 0.012 0.005 0.99 0.0410.0010 0.0041 — — B-23 0.70 0.49 1.54 0.011 0.006 0.51 0.112 0.00090.0034 — — B-24 0.60 0.25 0.77 0.009 0.007 0.83 0.025 0.0011 0.0036 — —B-25 0.66 0.20 1.01 0.011 0.005 0.99 0.022 0.0010 0.0041 — — B-26 0.690.16 0.99 0.012 0.005 0.77 0.035 0.0010 0.0033 0.22 0.43 B-27 1.05 0.250.45 0.016 0.008 1.45 0.025 0.0010 0.0031 — — B-28 0.64 0.25 0.86 0.0080.001 0.78 0.028 0.0009 0.0035 — — B-29 0.61 0.34 0.93 0.017 0.003 0.860.029 0.0008 0.0032 — — B-30 0.67 0.38 0.91 0.009 0.001 1.20 0.0250.0010 0.0035 — — B-31 0.67 0.34 0.89 0.011 0.002 0.40 0.033 0.00090.0035 — — B-32 0.80 0.26 0.91 0.010 0.003 0.95 0.027 0.0008 0.0042 — —B-33 0.60 0.25 0.79 0.011 0.004 0.85 0.027 0.0008 0.0020 — — B-34 0.610.26 0.81 0.011 0.004 0.88 0.023 0.0010 0.0160 — — Steel No. Mo W Nb TiZr V B Ec Note A-1 — — — — — — — −0.24 Reference steel B-1 0.33 — — — —— — 0.14 Inventive steel B-2 0.33 — — — — — — 0.17 Inventive steel B-30.45 — — — — — — −0.13 Comparative steel B-4 — — 0.012 — — — — 0.19Inventive steel B-5 — — — — — — 0.0012 0.10 Inventive steel B-6 0.41 — —— — — — 0.16 Inventive steel B-7 — — — 0.002 — 0.110 — −0.01 Comparativesteel B-8 — — — — 0.003 — — 0.17 Inventive steel B-9 0.41 — — — — — —0.27 Comparative steel B-10 — — — — — — — −0.14 Comparative steel B-11 —— — — — — — 0.11 Comparative steel B-12 — 0.53 — — — — — −0.03Comparative steel B-13 0.33 — — — — — — 0.12 Inventive steel B-14 0.15 —0.045 — — — — 0.06 Inventive steel B-15 — — — — — — 0.16 Inventive steelB-16 0.15 0.35 — — — — — 0.03 Inventive steel B-17 — — — 0.011 — — —0.09 Inventive steel B-18 — — — 0.011 — — — 0.20 Inventive steel B-190.28 — — — — — — 0.21 Inventive steel B-20 0.28 — — — — — — −0.04Comparative steel B-21 0.31 — — — — — — 0.13 Inventive steel B-22 — — —— — — — 0.20 Inventive steel B-23 — — 0.015 0.013 — — — −0.01Comparative steel B-24 — — 0.011 — — 0.055 — 0.17 Inventive steel B-250.29 — — — — — — 0.12 Inventive steel B-26 — — 0.011 — — — — 0.08Inventive steel B-27 — — 0.011 — — — — −0.24 Comparative steel B-28 0.26— — — — — — 0.13 Inventive steel B-29 0.32 — — — — — — 0.15 Inventivesteel B-30 — — — — — — — 0.11 Comparative steel B-31 — — — — — — — 0.07Comparative steel B-32 — — — — — — — −0.03 Comparative steel B-33 — — —— — — — 0.17 Comparative steel B-34 — — — — — — — 0.16 Comparative steel

TABLE 4 Billet Size after Casting and after Forging Heating Section ofbillet (mm) Section after forging (mm) temperature Steel No. Top BottomMaterial Shape*1 Size*2 (° C.) Note A-1 1350 × 1250 1280 × 830 Ingotsteel □ 550 1150 Reference steel B-1 1330 × 1230 1280 × 860 Ingot steel□ 700 1150 Inventive steel B-2 1100 × 1100  860 × 860 Ingot steel □ 6501100 Inventive steel B-3 1000 × 1000  700 × 700 Ingot steel ◯ 550 1150Comparative steel B-4 1250 × 1150 1180 × 730 Ingot steel ◯ 600 1150Inventive steel B-5 1450 × 1350 1380 × 930 Ingot steel □ 750 1100Inventive steel B-6 900 × 900  700 × 700 Ingot steel ◯ 450 1250Inventive steel B-7 1330 × 1230 1280 × 860 Ingot steel □ 600 1050Comparative steel B-8 1330 × 1230 1280 × 860 Ingot steel □ 600 1000Inventive steel B-9 1100 × 1100  860 × 860 Ingot steel □ 450 1150Comparative steel B-10 1450 × 1350 1380 × 930 Ingot steel □ 750 1050Comparative steel B-11 1450 × 1350 1380 × 930 Ingot steel ◯ 750 1150Comparative steel B-12 900 × 900  700 × 700 Ingot steel ◯ 450 1100Comparative steel B-13 1330 × 1230 1280 × 860 Ingot steel ◯ 800 1200Inventive steel B-14 1330 × 1230 1280 × 860 Ingot steel □ 700 1250Inventive steel B-15 1450 × 1350 1380 × 930 Ingot steel □ 600 1150Inventive steel B-16 1450 × 1350 1380 × 930 Ingot steel ◯ 600 1150Inventive steel B-17 1100 × 1100  860 × 860 Ingot steel □ 450 1350Inventive steel B-18 1250 × 1150 1180 × 730 Ingot steel ◯ 500 1000Inventive steel B-19 1100 × 1100  860 × 860 Ingot steel ◯ 400 1050Inventive steel B-20 1250 × 1150 1180 × 730 Ingot steel □ 550 1100Comparative steel B-21 1400 × 300  1400 × 300 Continuously casted steel□ 450 1150 Inventive steel B-22 1400 × 300  1400 × 300 Continuouslycasted steel □ 600 1200 Inventive steel B-23 1400 × 300  1400 × 300Continuously casted steel □ 450 1100 Comparative steel B-24 1400 × 300 1400 × 300 Continuously casted steel □ 550 1100 Inventive steel B-251400 × 300  1400 × 300 Continuously casted steel ◯ 650 1150 Inventivesteel B-26 1400 × 300  1400 × 300 Continuously casted steel ◯ 600 1150Inventive steel B-27 1400 × 300  1400 × 300 Continuously casted steel □550 1150 Comparative steel B-28 1330 × 1230 1280 × 860 Ingot steel □ 6501270 Inventive steel B-29 1330 × 1230 1280 × 860 Ingot steel □ 650 1250Inventive steel B-30 1330 × 1230 1280 × 860 Ingot steel □ 650 1250Comparative steel B-31 1330 × 1230 1280 × 860 Ingot steel □ 650 1250Comparative steel B-32 1330 × 1230 1280 × 860 Ingot steel □ 650 1250Comparative steel B-33 1330 × 1230 1280 × 860 Ingot steel □ 650 1250Comparative steel B-34 1330 × 1230 1280 × 860 Ingot steel □ 650 1250Comparative steel *1: ◯ denotes round forging, □ denotes square forging.*2: Diameter for round forging, Side length for square forging

Table 5 shows the presence or absence of eutectic carbide, the rollingcontact fatigue life characteristics, and the results of machinabilitytest. It was shown that steels B-1 to B-2, B-4 to B-6, B-8, B-13 toB-19, B-21 to B-22, B-24 to B-26, and B-28 to B-29, which satisfy ourcomposition and Ec contained no eutectic carbide in the steel and hadexcellent rolling contact fatigue life characteristics. In contrast,steels B-3, B-7, B-12, and B-23, which have a composition within ourrange, but Ec outside our range contained eutectic carbide in the steeland had a shortened rolling contact fatigue life. Steels B-9 to B-11,B-20, B-27, and B-31 to B-34, which have a composition outside ourrange, had a shortened rolling contact fatigue life. Steel B-30, whichhas Ec within our range, but a Cr content outside our range, hadinsufficient machinability.

TABLE 5 Test results Presence Rolling of contact Steel eutectic fatiguelife Tool life No. carbide Ec (Bn life ratio) ratio Note A-1 Yes −0.241.00 1.00 Reference steel B-1 No 0.13 1.16 1.21 Inventive steel B-2 No0.17 1.19 1.22 Inventive steel B-3 Yes −0.14 1.08 1.18 Comparative steelB-4 No 0.19 1.18 1.22 Inventive steel B-5 No 0.16 1.17 1.20 Inventivesteel B-6 Yes −0.01 1.09 1.22 Comparative steel B-7 No 0.17 1.18 1.23Inventive steel B-8 No 0.27 1.09 1.25 Comparative steel B-9 Yes −0.141.05 1.12 Comparative steel B-10 No 0.11 1.07 1.21 Comparative steelB-11 Yes −0.04 1.08 1.20 Comparative steel B-12 No 0.12 1.17 1.20Inventive steel B-13 No 0.05 1.13 1.19 Inventive steel B-14 No 0.16 1.181.23 Inventive steel B-15 No 0.08 1.13 1.20 Inventive steel B-16 No 0.091.13 1.21 Inventive steel B-17 No 0.20 1.12 1.17 Inventive steel B-1S No0.21 1.12 1.23 Inventive steel B-19 Yes −0.04 1.08 1.15 Comparativesteel B-20 No 0.14 1.17 1.23 Inventive steel B-21 No 0.13 1.15 1.23Inventive steel B-22 No 0.14 1.18 1.22 Inventive steel B-23 No 0.20 1.111.21 Inventive steel B-24 No 0.16 1.12 1.22 Inventive steel B-25 No 0.061.11 1.21 Inventive steel B-26 No 0.20 1.15 1.22 Inventive steel B-27 No0.16 1.19 1.22 Inventive steel B-28 No 0.06 1.15 1.21 Inventive steelB-29 No 0.10 1.14 1.12 Comparative steel B-30 No 0.07 1.08 1.21Comparative steel B-31 No −0.03 1.07 1.16 Comparative steel B-32 No 0.161.08 1.22 Comparative steel B-33 No 0.16 1.07 1.13 Comparative steel

INDUSTRIAL APPLICABILITY

Bearing steels having excellent rolling contact fatigue lifecharacteristics can be manufactured at low cost, and industrially veryvaluable bearing steels can be provided.

1. A bearing steel having a composition comprising: C: 0.56% by mass ormore and 0.70% by mass or less, Si: 0.15% by mass or more and less than0.50% by mass, Mn: 0.60% by mass or more and 1.50% by mass or less, Cr:0.50% by mass or more and 1.10% by mass or less, P: 0.025% by mass orless, S: 0.025% by mass or less, Al: 0.005% by mass or more and 0.500%by mass or less, O: 0.0015% by mass or less, N: 0.0030% by mass or moreand 0.015% by mass or less, and a remainder of Fe and incidentalimpurities, wherein eutectic carbide formation index Ec defined byformula (1) satisfies 0<Ec≦0.25, whereinEc=(−0.07×[% Si]−0.03×[% Mn]+0.04×[% Cr]−0.36×[% Al]+0.79)−[% C]  (1)wherein [ ] indicates the amount of component described in parentheses(% by mass).
 2. The bearing steel according to claim 1, wherein thecomposition further comprises one or more selected from the groupconsisting of: Cu: 0.005% by mass or more and 0.5% by mass or less, Ni:0.005% by mass or more and 1.00% by mass or less, and Mo: 0.01% by massor more and 0.5% by mass or less.
 3. The bearing steel according toclaim 1, wherein the composition further comprises one or more selectedfrom the group consising of: W: 0.001% by mass or more and 0.5% by massor less, Nb: 0.001% by mass or more and 0.1% by mass or less, Ti: 0.001%by mass or more and 0.1% by mass or less, Zr: 0.001% by mass or more and0.1% by mass or less, and V: 0.002% by mass or more and 0.5% by mass orless.
 4. The bearing steel according to claim 1, wherein the compositionfurther comprises B: 0.0002% by mass or more and 0.005% by mass or less.5. The bearing steel according to claim 2, wherein the compositionfurther comprises one or more selected from the group consisting of: W:0.001% by mass or more and 0.5% by mass or less, Nb: 0.001% by mass ormore and 0.1% by mass or less, Ti: 0.001% by mass or more and 0.1% bymass or less, Zr: 0.001% by mass or more and 0.1% by mass or less, andV: 0.002% by mass or more and 0.5% by mass or less.
 6. The bearing steelaccording to claim 2, wherein the composition further comprises B:0.0002% by mass or more and 0.005% by mass or less.
 7. The bearing steelaccording to claim 3, wherein the composition further comprises B:0.0002% by mass or more and 0.005% by mass or less.
 8. The bearing steelaccording to claim 5, wherein the composition further comprises B:0.0002% by mass or more and 0.005% by mass or less.